Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes than the conventional sources. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, green, and blue emitting LEDs. The ratio of the intensities of the various colors sets the color of the light as perceived by a human observer.
To replace conventional lighting systems, LED-based sources that generate light that appears to be “white” to a human observer are required. In principle, a white appearing source can be constructed from three narrow band light sources as described above. In fact, many different spectral combinations can be utilized to provide a light source that looks white when viewed directly by a human observer. Consider a white light source constructed from three narrow band LEDs. To a human observer, the light looks white and can be made to appear the same as a conventional source such as an incandescent lamp when the user looks directly at the light. However, when the two light sources are used to illuminate a scene consisting of colored objects, the results are markedly different. To reproduce the colors observed in a scene that is illuminated with the light source in a manner that matches the colors observed when the scene is illuminated with an incandescent light or sun light, the “white” light source must have a spectrum that is more or less constant over the visual wavelengths between about 400 nm and 600 nm. The narrow band LED sources do not have this property.
One method for utilizing LEDs to provide a broad-spectrum light source utilizes phosphors that convert the LED light to light having longer wavelengths in a broad spectrum. For example, a phosphor that emits light over a broad range of red wavelengths can be illuminated with UV from an LED that generates a narrow UV spectrum. The phosphor-generated red light is then used as a component of the white light source. By combining several phosphors, one can, in principle, create a broad-spectrum white light source that is suitable for replacing incandescent lamps provided the light conversion efficiencies of the phosphors are sufficiently high.
Unfortunately, a lamp designer does not have an arbitrary set of phosphors from which to choose. There are a limited number of conventional phosphors that have sufficient light conversion efficiencies. The emission spectrum of these phosphors is not easily changed. Furthermore, the spectra are less than ideal in that the light emitted as a function of wavelength is not constant. Hence, even by combining several phosphors, an optimum white light source is not obtained.
“Quantum dot” (QD) phosphors are phosphors whose emission spectra depends on the size of the particles, and hence, can be used to convert light to a predetermined wavelength by utilizing the appropriate sized particles. Quantum dots are semiconductor nanometer sized crystals. As the size of the particles decrease, the particles reach a size at which the band gap of the material becomes dependent on the particle size. As a result, the emission spectrum is shifted to smaller wavelengths as the particle size is decreased. For example, CdSe quantum dots emit light at various wavelengths in the visible region of the spectrum. The emission wavelength depends only on the particle size, and hence, a phosphor having the desired emission wavelength can be constructed by controlling the particle size.
Unfortunately, the emission spectrum of any given quantum dot phosphor is very narrow. In addition, the conversion efficiency of these phosphors is significantly lower than that of conventional phosphors. Hence, to provide a phosphor composition having the desired spectral shape, a very large number of quantum dot phosphors must be utilized, which makes this approach impractical for low cost incandescent or fluorescent light replacements. In addition, the low light conversion efficiency reduces the cost savings, as higher power UV LEDs must be used to drive the phosphors.